Problem: 9 binders cost $13.68. Which equation would help determine the cost of 15 binders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. If we let $x$ represent the cost of 15 binders, we have the following proportion: $\dfrac{x}{15}$ We have to pay $13.68 for 9 binders, and that can be written as a proportion: $\dfrac{\$13.68}{9}$ Since the price per binder stays the same, these two proportions are equivalent. $\dfrac{x}{15} = \dfrac{\$13.68}{9}$